Common Polymers used in Engineering (Leaving Cert Engineering): Revision Notes
Common Polymers Used in Engineering
Understanding different types of polymers and their properties is essential for engineering applications. Polymers can be classified into four main categories based on their structure and behaviour when heated: natural polymers, thermoplastics, thermosets, and elastomers. Each category has distinct characteristics that make them suitable for specific engineering purposes.
Natural polymers
Natural polymers are materials that occur naturally and have been used by humans for centuries. These polymers demonstrate that useful engineering materials can be found in nature.
Natural polymers have been humanity's first engineering materials, providing the foundation for understanding how molecular structure affects material properties. Their study helps engineers appreciate how nature has solved complex material challenges over millions of years.
Shellac is a hard and shiny polymer obtained from beetle dung. Its smooth, glossy finish makes it perfect for furniture polish and protective coatings. The natural hardness provides durability whilst the shine gives an attractive appearance.
Natural rubber comes from rubber tree sap and exhibits soft, flexible properties. This flexibility allows it to stretch and return to its original shape, making it ideal for elastic bands and other applications requiring elasticity.
Amber represents a fossilised natural polymer that has hardened over millions of years. Its translucent, hard properties make it valuable for jewellery, demonstrating how natural polymers can become extremely durable over time.
Thermoplastics
Thermoplastics are polymers that soften when heated and harden when cooled. This process can be repeated multiple times, making them highly versatile for manufacturing and recycling.
Key characteristic of thermoplastics: They can be reheated and reshaped multiple times without permanent chemical change. This reversible process makes them highly recyclable and ideal for manufacturing processes requiring multiple heating cycles.
Polyethylene (PE) comes in two main forms. Low Density Polyethylene (LDPE) is flexible and tough, perfect for plastic bags that need to bend without breaking. High Density Polyethylene (HDPE) is stronger and more rigid, making it suitable for lunch boxes that must maintain their shape under stress.
Polypropylene (PP) offers exceptional toughness and wear resistance. These properties make it ideal for products like blue rope and chairs that must withstand repeated use and outdoor conditions without degrading.
Polytetrafluoroethylene (PTFE), commonly known as Teflon, provides excellent heat resistance and non-stick properties. The smooth surface prevents food from sticking to frying pans, whilst the heat resistance ensures the coating remains stable at cooking temperatures.
Polymethylmetacrylate (PMMA), also called Acrylic or Perspex, combines versatility with corrosion resistance. Its tough yet workable nature makes it perfect for shop signs that must withstand weather whilst remaining clear and attractive.
Polyvinylchloride (PVC) offers versatility, low cost, and ease of working. These characteristics make it excellent for window frames, plumbing, and cable trunking where durability and weather resistance are important.
Polystyrene (PS) provides toughness and flexibility, with the ability to incorporate foaming agents. This makes it perfect for packaging and foam cups where lightweight insulation is required whilst maintaining structural integrity.
Thermosets
Thermosets are polymers that undergo permanent chemical changes when heated, forming crosslinks that cannot be reversed. Once set, they maintain their shape even when reheated.
Critical difference from thermoplastics: Thermosets form permanent crosslinks during heating that cannot be broken by reheating. This makes them dimensionally stable at high temperatures but prevents recycling through remelting.
Bakelite (Phenol-Formaldehyde) creates a hard, brittle plastic through crosslinking. This permanent hardness makes it suitable for frying pan handles and electrical components where heat resistance and dimensional stability are crucial.
Vulcanised rubber combines natural rubber with sulphur to promote crosslinking. This process creates a material that is extremely tough and wear-resistant, essential for car tyres that must withstand constant friction and varying temperatures.
Polyester can function as both a thermoplastic and thermoset depending on processing. In its thermoset form, it becomes hard-wearing and tough, making it ideal for plastic gears and brush bristles that require dimensional stability under stress.
Epoxy forms when resin and hardener are mixed, creating a hard and brittle material. This combination is perfect for adhesives and glues where permanent bonding and high strength are required.
Elastomers
Elastomers are polymers with elastic properties, meaning they can stretch significantly and return to their original shape. This elasticity makes them essential for applications requiring flexibility and resilience.
Elastomers possess unique molecular structures that allow long polymer chains to uncoil when stretched and return to their coiled state when the stress is removed. This property distinguishes them from both rigid thermoplastics and permanently crosslinked thermosets.
Natural rubber in its elastomer form maintains softness and flexibility whilst being produced from rubber tree sap. These properties make it perfect for elastic bands that must stretch repeatedly without losing their elasticity.
Silicone offers softness, flexibility, and waterproof properties. Its ability to maintain these characteristics across temperature ranges makes it excellent as a bathroom sealant where it must remain flexible whilst preventing water penetration.
Key Points to Remember:
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Four polymer categories: Natural polymers, thermoplastics, thermosets, and elastomers each have distinct properties and applications
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Thermoplastics can be reheated: Unlike thermosets, they can be softened and reshaped multiple times, making them recyclable
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Thermosets form permanent crosslinks: Once heated and set, they cannot be resoftened, providing dimensional stability
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Material properties determine applications: Flexibility for elastomers, heat resistance for PTFE, and hardness for Bakelite match their engineering uses
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Natural and synthetic options exist: Both natural polymers and manufactured polymers serve important engineering functions